Two researchers, Shawn Sederberg and Abdul-hakem Elezzabi, at the University of Alberta
(Edmonton, AB, Canada) have shown for the
first time that electrons excited via light in a
nanoscale silicon waveguide can be accelerated to energies of up to several electron volts
by the strong-field ponderomotive potential that exists in the highly confined plasmonic field. 1 One result is a strong white-light
emission from the silicon waveguide, even
though silicon is an indirect-bandgap material
and does not normally emit white light.

The nanoplasmonic silicon platform, which
is CMOS-compatible, operates in the 1550
nm wavelength region and occupies an ultra-compact square footprint that is 0.43 µm2.

The waveguides themselves are 100 nm wide
and 340 nm high and capped with a 60-nm-
thick gold film. They are excited by end-firing
laser pulses with an 84 ps duration into the
waveguides using a microscope objective; the
resulting electric fields range up to 5 V/nm.

Electron avalanche

As they propagate down the waveguide, the
femtosecond pulses excite free carrier electrons via two-photon absorption (TPA), which then accelerate
ballistically (some of them to energies above the impact-ionization energy Et) and eventually collide with valence-band electrons. If their energies are <Et when they collide, some electrons emit a photon. If their energies are >Et, they dislodge
another electron. The result is an electron avalanche, along with
wideband light emission between 375 and 650 nm and a third-harmonic signal that spans 500 to 530 nm (see figure). The
white-light output is collected into a single-mode optical fiber.

“The exponential growth of visible light emission confirms theexponential growth of the electron population, demonstratingthe presence of an optical-field-driven electron avalanche,” saysElezzabi. He notes that the impact-ionization process is similarto the phenomenon thatoccurs in avalanche-drivensemiconductor diodes,except for the fact that itis light and not an appliedelectric field that is acceler-ating the electrons.

The researchers also
showed via ultrafast
pump-probe time-domain
spectroscopy that such
electrons not only are
accelerated, but they are
also swept away inside
the semiconductor by the
nanoplasmonic field on a
time scale of the order of
a picosecond. This time
scale corresponds to the
expected sweeping time
defined by how long it
takes the electrons to travel
far enough from the gold/
silicon plasmonic interface
that they stop interacting
with the probe.

“The high sensitivity of the electron avalanche process will
enable the development of compact, sensitive optical circuitry
and interfacing between nanoplasmonic and electronic components,” says Elezzabi. “These findings offer a means to harness
the potential of the emerging field of nonlinear nanoplasmonics.
To our knowledge, the experimental demonstration of electrons
gaining energy form an optical field in semiconductors presents
new physics that will open up new opportunities for nanoplasmonic devices.” — John Wallace

Nanoplasmonic waveguides are excited by ultrafast pulses
with a 1550 nm center wavelength, producing an electron
avalanche and white-light emission (microscope image,
top left). The emitted light has a continuous spectrum
ranging from 375 to 650 nm, along with a spike at the third-harmonic-generation wavelength region (top right). The
calculated electron-energy spectra are shown for ultrafast-pulse-produced electric fields of 1, 3, and 5 V/nm (bottom).
(Courtesy of the University of Alberta).